Vegetable one-bath pretanning process



3,081,142 VEGETABLE QNE-BATH PRE'IANNING PROCESS Ernesto Mezei, Tapinerita 1%), Barcelona, Spain No Drawing. Filed Mar. 9, 1%1, Ser. No. 94,422 Claims priority, application Germany Mar. 12, 1960 5 Qiaims. (Cl. 8-94.1)

This invention relates in general to a process for tanning sole-leather and in particular to a new and useful process in which pretanning is ellected on delimed pelts using a buttered tanning liquor in a single operation.

In every process for tanning sole-leather, the hide must be prctanned in order to separate the fibers, to let the tannin penetrate thoroughly and to avoid the so-called hard-casening effect upon the leather. This pretanning process requires a large number of tanning pits as Well as a large volume of weak tanning liquors of density 1 to 6 es. With cattle hides the duration of the pretanning may be several days or weeks. This process is very slow, uneconomical and needs many working hours. Very much tannin is lost by precipitation, oxidation, fermentation and also in the sewage. Besides the hides, immersed in the pits, and the tannin extract dissolved require high capital investments.

The above procedure is known as pit tonnage. Many attempts have been made to improve the procedure, such as by variations of the concentration, the temperature, the pH value and the movement of the liquors or the hides between Wide limits of movement in order to accelerate the overall tanning process, pretanning with syntans (that is, synthetic tannins) or adding to the liquors syntans, lignosulphonics, tensoactives, etc. In addition, accelerators such as formal, chrome or aluminum salts with or without a combination of syntans were used on delimed or pickled material. In such cases, however, mixed tannages are obtained with other properties or leather which have neither the firmness nor the yield which are necessary for sole-leather.

in accordance with the present invention, it was found that it was possible to obtain a very quick, simple and satisfactory pretannage on the normally delimed pelts by using a buffered tanning liquor in a single operation. In a preferred process, in accordance with the invention, the treatment of the pelt delimed at a pH of 6 is carried out in a single treatment lasting less than 24 hours in a drum with a 100% solution, calculated on pelt-weight, containing bullered tanning liquors of a pH 6-8.

This buffered tanning solution penetrates thick cattle hides within 24 hours and gives a long fibered, perfectly smooth leather of a very light color, completely uniform section, without any oxidation and with remarkable fullness. This pretanned material can be retanned by any of the classical methods of vegetable tanning to be used for sole-leather, split hides, upholstery, etc., resulting always in a correct and normal end product. The pretanned leather could also be dried and can be used as crust with a low soluble matter and for various purposes.

Accordingly, it is an object of this invention to provide an improved pretanning process for treating pelts.

A further object of the invention is to provide a pretanning process for treating pelts in a single operation lasting no more than 24 hours.

A further object of the invention is to provide a pretanning process for treating pelts in a single operation including treating at pH 6 delimed pelts in 100% tanning solution calculated on pelt weight which is buffered to a pH of 6-8.

A further object of the invention is to provide a process for pretanning pelts which is simple to execute, requires a minimum amount of materials and expense and may be accomplished in a relatively short period.

50 g. tannin per liter.

fidijdz Patented Mar. 12, 1963 While specific embodiments of the invention will be disclosed herein and examples given, those skilled in the art should appreciate that the invention may be embodied or carried out otherwise without departing from the scope of the invention.

In accordance with the invention, it is possible to provide a pretanned leather having very advantageous characteristics by insuring that the following conditions are preferably observed:

(1) A commercial tanning extract such as sulphited quebracho-, chestnut-, wattle, vallonea-, etc. extracts or mixtures are dissolved instead of in water in a buffer solution. This solution should not contain more than The buffer solution contains 40 to g. dry salts per liter.

(2) The pI-l values of these bufiered liquors should be between 65-82, preferably pH 7.

(3) The ratio of buffer-salts to tanning agent must be determined in such a manner that the respective agent is brought to the limit of its flocculation point without, however, reaching that point and thus becoming flocculated.

(4) The temperature of the tanning liquors and during the Whole tanning process must be maintained at 35 C.

Under these conditions, in a slow turning drum with only 100% of bath liquor calculated on pelt weight and in no more than 24 hours, a completely pretanned and full leather is obtained. The required quantity of tannin (12 to 14%) is added by portions to the butter solution maintaining the tanning concentration around 5%.

As butter salts preferably ammonium sulphate or ammonium chloride, sodium sulphite or bisulphite, sodium carbonate or sodium bicarbonate, mono-, bior tri-sodiurn phosphate or polyphosphates, sodium thiosulphate, boric acid and borates are used. Also organic acids as acetic, formic, lactic, citric, tartaric, thioglycolic, amino acetic, etc., may be used or the ammonium salts of them or their combination with Weak bases as triethanolamine, pyridine, etc.: also could be used sulphonic acid salts of napthalene, phenol or cresols, lignosulphonic acid salts, syntans of every class, detergents, etc. important is the buffering eliect of all thesecompounds which naturally is combined with the tensoactive or tanning effects of the other components. But the pH must always be between pH 6 to 8. It has to be remarked that the lignosulphonates and syntans by themselves are ineffective with this pH range.

As above mentioned the butter salts have to be adapted separately for each type of tanning agent or their mixtures. It is of importance that the amount of tannins is chosen so that it is just below the salting-out point of the tannins and that no oxidations nor flocculations occur, having observed that the presence of sulphites or other reducing agents is very useful to this purpose.

One form of the process is, practically, as follows: In a big drum, which rotates slowly, 100% water calculated on pelt weight is put in, at 38 C., and the buffer salts dissolved in it; the calculated amount of tanning ex tract, pulverized (5% tannin) are added; the pelts are joined and drummed during 3 to 4 hours; this liquor has at this moment a density of 8-10 B., the temperature of 35 C. and a pH of 7.2. To complete the pretanning 7-9% pure tannin are needed or 10-13% tannin extract, which is added every 2 to 3 hours according to the exhaustion of the bath. After 24 hours of drumming the pelt is completely tanned and the liquor exhausted nearly to 1% tannin. It was used totally, around 20% tannin extract or 12-14% pure tannin. Now the bath is drained (at temperature around 28 C. and pH 6.5) and the stock is rinsed with cold water in order to eliminate the buffer salts.

This simple procedure possesses notable advantages compared with the old pit tanning if it is considered that only 30 liters of tanning liquor per hide are used for the pretanning instead of 1500 liters. Also that only 0.2 working hour is needed instead of 3.3 as in the old method; that for the same tanning degree only 14% pure tannin is required instead of 17%, that only 30 liters exhausted liquor per hide are drained instead of 120 liters; that 10 to 15 times less space is needed and that the whole manipulation of liquors, pumping, reinforcing and cleaning is facilitated- This surprising tanning effect is presumably due to the high osmotic pressure of the liquors, to exchange of ions, to the molecular dehydration of the hide-proteins and of the tanning miscelles and to the displacement of the isoelectric point. Furthermore a hydrotropic swelling is observed similar to the leathers pretanned at pH 3.5, an eifect which was surely not forseen at pH 7, and in the presence of so many salts.

The following examples help to illustrate the process:

(1) 100 pelts of cattle, delimed at pH 6 and weighing 3000 kg. are put in the drum (3 to 4 r.p.m.) and pre-' tanned in the following bath: 3000 liters water at 38 C. in which there are dissolved 30 kg. sodium naphthalenesulphonate, 30 kg. sodium hexa-meta-phosphate, 30 kg. sodium thiosulphate, 105 kg. sodium sulphite, 18 kg. sodium acetate, 12 kg. potassium monophosphate. This solution has a pH 7.6. Now are added 240 kg. of pulverized quebracho extract. The pH falls to 7.2. The stock is loaded and runs during 3 to 4 hours. Afterwards are added every two hours in portions of 50 kg., 450 kg. of the same quebracho extract, taking much care that the amount of pure tannin never passes 5% (colorimetric control). the exhaustion is nearly completed. In around 24 hours the pelts are fully penetrated,being smooth, light colored and plump. They are rinsed with running water during,

5 to minutes and the pretannage is achieved.

(2) 100 pelts of cattle hides, which are destinated for sole-leather (peltweight 3000 kg.) are treated in the drum the same way as in Example 1) (pl-I 6). The drum contains 3000 liters of water, at 38 C., in which are dissolved: kg.,boric acid, 12 kg, triethanolamine, 90 kg. sodium sulphite, 12 kg. lactic acid and 6 kg. ammonium lactate. The pH is 7.2. Then 240 kg. of sweetened chestnut extract (pH 4.2) are added, so that the bath has a pH of 6.4. The stock is loaded and are running during 3-4'hours. Afterwards are added every 2 hours in, portions of 50 kg., 450'kg. of the same chestnut extract. After the last addition the stock runs until After the last addition the stock runs until previously delimed at a pH of about 6, which comprises,

placing the pelts in a slowly revolving drum with an aqueous tanning liquor, the volume ratio between said liquor and said belts being about 1:1, sa-id liquor containing an amount of buffer salts sufficient constantly to maintain the pH value of the liquor within a range of about 6-8, said liquor further comprising vegetable tanning agents in an amount not exceeding 5% of pure tannin by weight, and gradually adding tanning agents to the liquor so as to maintain the pure tannin concentration of the liquor at about 5% and at a level just below the flocculation point of the tannin and maintaining the temperature of the liquor at about 35 C., whereby complete pretanning of the pelts is effected in about 24 hours. 2. A pretanning process according to claim 1, wherein salts, tensoactives, polymers, phosphates, syntans are added to the butter salts.

3. Apretanning process according to claim 1, including adding reducing salts for buffering.

4. In a one-bath vegetable pretanning process for delimed pelts, wherein the pelts, delimed at a pH of about 6, are placed in a revolving drum and are pretanned by a vegetable tannin containing aqueous liquor, the improvement which comprises that a. the volume ratio of liquor to pelts is about 1:1, 17. the aqueous liquor is buffered by buffer salts dissolved in the liquor to a pH ranging between 6 and 8, and

c. the amount'of tannins in the liquor is constantly maintained at a value of at the most 5% by weight of pure tannin calculated on the weight of the liquor and is close to but below the flocculation point, whereby pretanning in about 24 hours is effected.

5. The improvement of claim 4, wherein the amount of buffer salts in said liquor is maintained at about 0.4 to 10% by weight calculated on the weight of the liquor.

References Cited in the file of this patent UNITED STATES PATENTS 2,140,042 Wilson Dec. 13, 1938 2,437,044 Robinson et a1. Mar. 2, 1948 2,804,367 Moore et al Aug. 27, 1957 2,805,117 Perrin. Sept. 3, 1957 

1. A ONE-BATH VEGETABLE PERTAINING PROCESS OF PELTS PREVIOUSLY DELIMED AT A PH OF ABOUT 6, WHICH COMPRISES PLACING THE PELTS IN A SLOWLY REVOLVING DRUM WITH AN AQUEOUS TANNING LIQUOR, THE VOLUME RATIO BETWEEN SAID LIQUOR AND SAID BELTS BEING ABOUT 1:1, SAID LIQUOR CONTAINING AN AMOUNT OF BUFFER SALTS SUFFICIENT CONSTANTLY TO MAINTAIN THE PH VALUE OF THE LIQUOR WITHIN A RANGE OF ABOUT 6-8, SAID LIQUOR FURTHER COMPRISING VEGETABLE TANNING AGENTS IN AN AMOUNT NOT EXCEEDING 5% OF PURE TANNIN BY WEIGHT, AND GRADUALLY ADDING TANNING AGENTS TO THE LIQUOR SO AS TO MAINTAIN THE PURE TANNING CONCENTRATION OF THE LIQUOR AT ABOUT 5% AND AT A LEVEL JUST BELOW THE FLOCCULATION POINT OF THE TANNING AND MAINTAINING THE TEMPERATURE OF THE LIQUOR AT ABOUT 35* C., WHEREBY COMPLETE PERTAINING OF THE PELTS IS EFFECTED IN ABOUT 24 HOURS. 